A noisy furnace blower usually indicates a problem with either the blower's electric motor or the blower wheel. Turn the furnace off, disconnect it from both the electricity and gas, wait for it to cool completely and then dismantle the blower assembly, and inspect it for problems. Hire a professional to resolve the problem if you do not have the necessary training, as making inadequate repairs to a broken furnace blower can have extremely dangerous consequences.
High, shrill sounds usually indicate that two metal surfaces are grinding against one another. Because high-speed contact between two metals can cause sparks, these sounds require immediate attention. These sounds are sometimes caused by the blower wheel coming loose from the blower motor's shaft and scraping against the internal housing. This problem is relatively easy to fix, as it only requires reattaching the wheel to the shaft. These sounds can also indicate a broken wheel or motor, with both problems requiring replacement parts.
Thumping or bumping noises usually indicate that the blower wheel is unbalanced or coming loose from its shaft. Fixing these problems typically only requires tightening the wheel to the shaft.
A loud humming noise usually indicates an electrical problem. The problem could be with the blower motor or an electrical transformer in the furnace. Some transformers and motors are, unfortunately, loud even when functioning normally, but a loud humming sound that arises suddenly requires attention.